(A) Field of the Invention
The present invention relates to a manufacturing method of an over-current protection device, and more particularly, to a method for manufacturing an over-current protection device having positive temperature coefficient (PTC) conductive composite material.
(B) Description of the Related Art
Because the resistance of conductive composite materials having a positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (e.g. at least 104Ω), which is the so-called trip. Therefore, the over-current will be eliminated so as to protect the cell or the circuit device.
The manufacturing of a low resistance (volumetric resistance<0.1 Ω-cm) PTC over-current protection device is generally performed as follows. First, crystalline polymer, e.g., high density polyethylene (HDPE) or low density polyethylene (LDPE) and oxygen-free conductive ceramic powder (e.g., titanium carbide) are mixed using, for example, a Hakke mixer at 50 rpm and 160° C. for 15 minutes to form a PTC material. The PTC material is then put into a hot presser. A steel plate and Teflon mold-release cloth are disposed at top and bottom surfaces of the PTC material and pressed at 180° C. to form a PTC laminate. Sequentially, two electrode foils are disposed at top and bottom surfaces of the PTC laminate, and the combination is pressed to create a PTC composite material, i.e., a structure of electrode foil/PTC laminate/electrode foil, of a thickness between 0.45 and 0.65 mm. The PTC composite material is punched into a plurality of chips (current protection devices) of around 2.8 mm×3.5 mm. Table 1 shows the initial resistances, sizes and the resistance after 10 cycles of life test of twelve samples of the over-current protection devices made according to the above method. A cycle of the life test is to apply 12 volts and 10 amperes to the over-current protection device for 10 seconds followed by 60 seconds with no current. The initial resistances ranges from 0.0101Ω to 0.0195Ω, while the standard deviation is 0.003. In low resistance applications, the initial resistances in Table 1 vary drastically; therefore the distribution needs to be improved.
TABLE 1Resistance afterInitial ResistanceThickness10 cyclesSample(Ω)Width (mm)(mm)of life test (Ω)10.01802.810.750.025120.01462.830.730.027630.01942.820.740.025840.01632.840.740.030150.01952.840.750.019260.01652.810.730.022670.01242.840.730.018480.01012.830.730.018990.01352.830.710.028310 0.01402.850.700.020911 0.01192.850.690.020812 0.01162.850.670.0221Mean0.01482.83330.72250.0233Min.0.01012.81000.67000.0184Max.0.01952.85000.75000.0301Standard0.00300.01370.02380.0038Deviation
For high voltage (over 250V) applications of the PTC over-current protection device, the manufacturing method is similar to that of the low resistance PTC over-current protection device, and is familiar to those skilled in the art. The only differences are changes in the composition and percentage of PTC material. For example, high density polyethylene (HDPE), magnesium hydroxide and carbon black are used. In order to withstand high voltages, the thickness of the device is greater than that of low resistance PTC over-current protection devices. Therefore, high voltage PTC over-current protection devices manufactured according to the above method have non-uniform initial resistances. Table 2 shows initial resistances and thicknesses of fifteen samples of PTC over-current protection devices for high voltage applications, in which the distribution of the initial resistances, with standard deviation of 1.6279, is larger than that shown in Table 1 (with standard deviation of 0.003).
TABLE 2Initial ResistanceThicknessSample(Ω)(mm)16.833.5827.753.5636.933.5947.583.5654.553.4666.93.54710.183.5789.943.5796.033.5210 9.643.5611 8.423.5312 7.833.5113 5.893.5914 7.233.5815 5.553.51Mean.7.41673.5487Min.4.553.46Max.10.183.59Standard Deviation1.62790.0366